Overview of Ecology of Coral Reefs in Hawai,'i

A. Uniqueness of Hawaiian Ecology

The ecology of coral reefs in Hawai'i is unique compared to reefs in other
tropical areas. Hawaiian reefs are unique for several reasons:

1. They are geologically young and therefore not as well developed
as other reefs. Most reefs in the windward islands of Hawai'i, Maui, O'ahu and Kaua'i are
small and occur relatively close to shore. As these reefs grow and develop they are called
fringing reefs, which eventually develop into barrier reefs, which occur in Kaneohe Bay on
O'ahu and on the south shore of Moloka'i. There is a general westward trend towards
greater reef development which coincides with the geologic ages of the islands.
Superimposed upon this pattern, however, is the effects of wave exposure: in general more
sheltered leeward coasts have reefs with greater coral cover than wave-pounded windward
coasts (see Grigg, 1983).

Narrow coral reef at Puako on the Big Island, typical of young
Hawaiian coral reefs

2. As a consequence of their younger age and the general absence of barrier reefs in the
windward islands, Hawaiian reefs are generally less productive than other reefs. With
smaller reef areas and the absence of lagoons that trap and retain both coastal and
terrestrial runoff, Hawaiian reefs are generally more nutrient poor and therefore lack a
high abundance of filter feeding animals such as soft corals, sponges, tunicates, and
bivalves. As a result, Hawaiian reefs are more clearly dominated by corals.

3. Due to the geographic isolation of the Hawaiian Islands they are less diverse that
other reefs, especially those in the nearby Indo-Pacific province to which they belong.
For example, in Hawai'i there are about 40 species of reef building corals; in the western
Pacific island of Palua there are over 300! One consequence of lower diversity is that
reef-building corals in Hawai'i are less specialized and thus more generalized in their
distribution than species elsewhere. Furthermore, a relatively few number of corals
dominant Hawaiian reefs. There is a unique advantage to science here, however, because
Hawaiian reefs are far easier to study!

B. Coral Reef Zonation in Hawai'i

In response to variation in environmental conditions near the shoreline, coral
reefs exhibit zonation where the abundance and composition of the coral community varies
according to distance and depth from shore. Wave exposure is the primary factor causing
zonation in Hawai'i but gradients in sedimentation, salinity, and temperature are also
important (see Dollar, 1982). The following is
a summary of zonation patterns typical of Hawai'i. Common names are linked to descriptions
in the Marine Life of Hawai'i section.

Reef Zonation Patterns

Reef flat zone
(0-2 m)

Reef bench zone
(2-10 m)

Corals are generally sparse here and dominated by
cauliflower coral
intermixed with boulders.